Project agent status into chrome.storage via a statuschange listener
instead of pairing setup/teardown across lifecycle hooks. A throwing hook
can no longer leak the heartbeat or strand isAgentRunning, and rejected
concurrent execute() calls never touch the active run's state.
- add `stop` method. agent can be reused after stopped
- agent can not be reused after disposed
- extension DO NOT exposes `dispose` anymore. only `stop`.
- update panel for new `stop` method
- fix MultiPageAgent dispose event
- better handling abortSignal
Simon